Ultrasonic dental scalers are generally used to clean patients' teeth and for other ultrasonic procedures such as general supra and sub-gingival scaling applications, periodontal debridement for all types of periodontal diseases, and endodontic procedures, for example. Dental scalers include a control circuit and a handpiece having an ultrasonic transducer, an energizing coil, magnetostrictive stack and a tool tip. In operation, the energizing coil surrounds the magnetostrictive stack and is energized by engaging a foot pedal that engages the control circuit to provide an electric current to the energizing coil. This, in turn, actuates the ultrasonic transducer by activating a stack of plates of magnetostrictive material that expands and contracts when subjected to a time-varying electro-magnetic field generated by the energizing coil in response to the electric current. In particular, the time-varying electro-magnetic field is created by directing a time-varying electric current through the excitation coil surrounding the magnetostrictive stack which causes the tool tip to vibrate at the resonant frequency of the ultrasonic transducer. The vibrating tool tip is then used by the dental practitioner to clean a patient's teeth by, for example, removing plaques and other debris from the surface of the patient's teeth.
Vibration of the tool tip is controlled and adjusted as appropriate during operation to tune the frequency and amplitude of the electric current applied to the transducer to a desired optimal operational frequency and amplitude of the ultrasonic transducer. As operational conditions change, such as load on the tool tip, temperature, density of the material being removed, and the like, the operational frequency and amplitude change accordingly and it becomes necessary to adjust the time-varying electric current to, in turn, adjust the time-varying electro-magnetic field to maintain the desired optimal operational frequency and amplitude. For this purpose, automatic frequency and amplitude tuning circuits have been developed in the prior art that use feedback coils, phase locked loops, and the like, to adjust the frequency and amplitude during operation to the resonant frequency of the natural acoustic modes of vibration of the magnetostrictive stack to optimize the vibration energy applied to the tool tip.
For example, U.S. Pat. No. 6,241,520 discloses an automatically tuned drive circuit for driving an ultrasonic scaling probe at a desired frequency of operation based on the choice of scaler insert for the handpiece. The oscillator of the drive circuit is coupled to the energizing coil in the handpiece for applying an oscillatory current to the magnetostrictive element. The drive circuit includes a frequency detector for sensing the frequency of the magnetostrictive element, and the detector's output signal further designates the magnitude of the frequency. The drive circuit responds to the sensed value of the frequency amplitude and adjusts a current applied to the energizing coil to adjust the oscillation frequency commanded by the scaling probe.
U.S. Pat. No. 7,614,878 similarly discloses a system for dynamically controlling an ultrasonic magnetostrictive dental scaler by providing a control circuit including a digital signal processor that processes sensed feedback signals regarding frequency and amplitude of the vibrations and filters the signals through dynamic filter loops to obtain error and/or control signals to adjust a voltage controlled oscillator that, in turn, controls the amplitude and phase characteristics of the time-varying electric current applied to the energizing coil. Varying the amplitude and phase of the time-varying electric current controls the output of the dental scaler so as to control the frequency and amplitude of the vibrations of the dental scaler to the desired operating point.
U.S. Pat. No. 6,503,081 discloses the use of a microprocessor in the drive circuitry to set the frequency of oscillation such that the power delivered to the excitation coil is maximized. The microprocessor is programmed to sense the power input to the excitation coils and the voltage-current phase difference measurements or power response slope measurements are used to determine the maximum power transfer point in order to set the oscillation frequency to the resonant frequency of the magnetostrictive insert.
U.S. Pat. No. 7,715,167 also discloses a control unit for setting the frequency of the excitation current flowing in an excitation coil of a magnetostrictive ultrasonic dental device. The control unit employs a voltage-controlled oscillator (VCO) that generates a variable frequency signal, a driver for setting up and regulating the excitation current according to the variable frequency signal from the VCO, a current sensor in series with the excitation coil that outputs a current-sense signal corresponding to the current flowing through the excitation coil, a functional block that receives the current-sense signal and outputs a function signal proportional to the sensed current, and a microprocessor that receives the function signal and controls the VCO according to the function signal. This control unit differs from other prior art control units in that even though the feedback which controls the frequency of oscillation is solely in connection with the sensed current passing through the excitation coil, the excitation coil is not part of the VCO circuit and is not directly connected to the VCO circuit.
The excitation coil in the handpiece is electrically connected via a cable to the control unit that provides the excitation energy to the excitation coil. The cable typically includes two input wires connected across the terminals of the excitation coil for driving the excitation coil as well as a third wire that provides the current-sense feedback to the control circuit for use in adjusting the current to the excitation coil to maintain the optimal operational frequency as discussed above. Unfortunately, the third wire adds significant weight to the cable, which increases cable drag that increases strain on the clinician's hand, wrist, and forearm during use. Also, the feedback control loop complicates the circuitry needed to maintain the operation of the ultrasonic transducer at the desired optimal operational frequency for optimal operation, and the circuitry is generally inefficient, generates too much heat during operation, and accordingly, does not permit the device to be as compact as desired.
It is desirable to provide an ultrasonic magnetostrictive driver that does not require the feedback wire and thus may support a lightweight cable and that is small and efficient to ensure that the magnetostrictive transducer is operating at the desired optimal operational frequency without generating excess heat. The invention addresses these and other needs in the art.